Bromonapthalenes have no known industrial use or application, but have identified as contaminants of Firemaster BP-6, the toxic mixture of polybrominated biphenyls used as a fire retardant and involved in a major episode of environmental poisoning in Michigan. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the position of bromination. This work has studied the chemical disposition of a mixture of 2-hexabromonapthalenes (HBNs), previously identified as a single isomer, 1,2,3,4,6,7-HBN. The compound is incompletely absorbed after an oral dose. After iv treatment over 50% of the dose is excreted as metabolites within 3 days. However, the remainder of the dose seems to be extremely persistent, over 25% remaining in the liver after 35 days. These disposition results led to proof of the presence of two isomers by high resolution NMR, present in a ratio of 60:40 which have been tentatively identified as 1,2,3,4,6,7- and 2,3,4,5,6,7-HBN. The difference in the fate of the two isomers has been proven by isolation and characterization of the HBN remaining in the liver 10 days after treatment. While the treatment of HBN was in the isomeric ratio of 60:40 (1,2,3,4,6,7: 2,3,4,5,6,7), the HBN in the liver 10 days after oral treatment was in the ratio of 20:80. The toxicity of this HBN mixture is also being studied. The acute toxic dose response is being defined in mice. This will be followed by an analysis of the teratogenicity of HBN, both alone and in combination with polybrominated biphenyls.